This invention pertains to a pressurized cyclonic combustion method and burner apparatus for effective combustion of particulate solid fuels to produce clean pressurized hot effluent gases. It pertains particularly to a cylindrical pressurized burner which utilizes helical flow patterns to provide prolonged combustion of the fuel solids and uses an intermediate choke zone and an air quench step to improve combustion and control temperature of the effluent gases produced.
Various types of burners for particulate solid fuels have been previously proposed such as disclosed by U.S. Pat. No. 2,614,513 to Miller et al; U.S. Pat. No. 2,769,411 to Simmons; and U.S. Pat. No. 2,881,720 to Lotz which utilize tangential swirling flow patterns and a restricted exit opening intended to retain solids in the burner longer for more complete combustion. U.S. Pat. No. 3,199,476 to Nettel discloses a similar burner for coal having dual tangential inlet for the small and coarser coal particles, a restricted throat exit for combustion gases and a lower drain port for slag removal. Other similar burners have been disclosed by U.S. No. 3,244,220 to Kloecher; U.S. Pat. No. 3,453,976 and U.S. Pat. No. 3,472,185 to Burden et al, but they do not have restricted outlets and are not intended for pressurized operations. U.S. Pat. No. 3,777,678 and U.S. Pat. No. 4,033,505 to Lutes et al disclose a horizontal cyclonic type burner for combustible solid materials in which the fuel is introduced tangentially into the combustion chamber at its inlet and combustion air is introduced tangentially along the length of the burner, which has a restricted choke outlet. Also, U.S. Pat. No. 4,422,388 to Raskin discloses a horizontal cylindrical burner for solid fuel introduced tangentially at one end, but maintains a fluidized bed of fuel in its lower portion. In addition, scroll or dual register horizontal fired type burners such as the Coen DAZ burner have been used for the combustion of the air conveyed solid fines. Such burners have dual registers with concentric louvers which in effect divide the air stream into two counter-rotating concentric streams which scrub against each other and provide turbulent mixing action for the fuel introduced into the annular space between the dual air streams.
It is noted that these prior art burners are useful for burning particulate solids at essentially atmospheric pressure for incineration and also for the recovery of heat energy. However, further improvements are needed in combustion of particulate solid fuels at above atmospheric pressure and in burner design for achieving higher throughputs for the fuels and higher heat release rates in the burner to produce relatively solids-free pressurized hot effluent gases suitable for power recovery applications.
Burning solid fuels, unlike burning vaporous and volatile liquid fuels, require increased reaction time constants which are orders of magnitude longer for complete combustion, i.e., mass diffusivity is rate controlling in the rapid oxidation of solid fuels. Historically, this longer combustion time requirement has been minimized by solids size reduction, as in firing pulverized coal instead of chunk or briquette coal. While such size reduction is beneficial, it still does not permit the firing of solid fuel materials at combustion rates which approximate those attained for non-solid fuels. This difference is most apparent when volumetric energy releases for various heat generators are compared.
In order to increase the fuel particle retention time in a burner, which time varies inversely with combustor size for a given heat release, a new method for achieving increased dual phase residence time for the solid fuel particles has now been developed. In this method, the solid fuel particles having higher mass are retained for very long periods of time relative to the lower mass combustible volatiles and gaseous materials in a cylindrical combustion chamber having an aspect ratio of longitudinal length more than about twice that of the chamber inside diameter. Into this combustion chamber the particulate solid fuel is introduced tangentially in lean phase transport near the inlet end. The fuel tangential velocities in the burner are sufficient so that very high centrifugal forces are imposed on the fuel particles which are swirled around the inner periphery the burner, while the gaseous material not as subject to such centrifugal forces and moving by molecular motion is free to move along the burner longitudinal axis while rapidly combusting, and then escape through a modulating restriction opening at the burner exit end.
The solid fuel particles are maintained in this helical flow pattern, trapped by their relatively high mass and high rotational velocity, slowly moving helically and at high Reynolds number condition toward the outlet end of the combustion chamber. This extended combustion path is significantly prolonged by the tangential injection of combustion air along the longitudinal axis of the burner at a high velocity. This combustion air is introduced under conditions of high tangential velocity and associated high Reynolds number, so as to impart an additional tangential acceleration to the fuel particles sufficient to overcome any reduction in velocity due to flowing resistance of the orbiting particles. Accordingly, this combustion process is continued under high Reynolds number conditions until the fuel particles are sufficiently destructed to produce gaseous products which escape the centrifugal forces in the combustion chamber and pass out at the burner exit end.
The gas residence time in the combustion chamber is a function of the volumetric throughput only, however, the fuel solids residence time is pathway dependent and is determined by the combustion chamber circumference and the number of revolutions divided by the velocity of the particles. Therefore, a particle can be retained in the burner to destruction for a much longer time than can volatile fuels and products of combustion which accompany the particle.
This improved combustion method also promotes more rapid combustion of solid fuels by forcing the circulating particles closely adjacent to the hot radiating interior surfaces of the combustion chamber. This radiating surface is quite large relative to the particle and served to sustain the reaction by constantly providing the threshold energy needed for combustion. The addition of the acceleration air along the helical path of the fuel particle promotes rapid oxidation by continually supplying fresh oxygen very near the particulate fuel solids undergoing reaction.
The addition of combustion air tangentially along the length of the combustion chamber also provides an additional benefit to the mechanical integrity of the burner by buffering and tempering the surface of the heat resistant refractory insulation material lining the interior of the burner chamber from the very hot products of combustion. This air addition prevents the insulation surface from reaching reaction temperatures which would be deleterious to the insulation. This air-sweep is enhanced by the design and installation of the air inlet tuyeres.
Accordingly, it is an object of the present invention to provide a pressurized cyclonic combustion method and burner apparatus for particulate solid fuels which provides for prolonged combustion of the fuel particles at conditions of high tangential velocities, high centrifugal forces, and high Reynolds numbers. Another object is to provide such a burner which operates at highly turbulent conditions and high Reynolds numbers and provides very high volumetric heat release rates approaching those for liquid and gaseous fuels. Another object is to provide a burner for solid particulate fuels such as wood chips which produces a clean hot pressurized effluent gas stream suitable for use in power producing processes.